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This paper proposes a subwavelength hierarchical honeycomb structure (SHHS) with a compact lateral dimension and double-band perfect absorption in low frequencies. Unlike the conventional micro-perforated panel (MPP)-honeycomb sandwich absorber, this structure has an additional internal honeycomb with a perforated wall. Therefore, there are two resonant cavities in the SHHS to realize multiple absorption peaks. Analytical, numerical, and experimental investigations are performed to study the proposed system's acoustic performance in absorption. The SSHS is simplified into four parts and its analytical model is constructed by combining various analytical models by acoustic-electro analogy. The analytical model is presented to explore the physical properties of sound absorption and the influence of parameters, which has been validated by comparisons with the numerical model, and the experimental data is measured by an impedance tube. It is found that the main incident energy is lost by the inside hole, which is different from the conventional absorbers with surface MPP. Moreover, the side length of the internal honeycomb can adjust the resonant frequencies to achieve an absorber with the subwavelength. A SSHS is designed with a perfect absorption at 320 Hz whose thickness is 1/31 of the resonant frequency wavelength. The SHHS has excellent potential for noise control engineering applications. [ABSTRACT FROM AUTHOR]

The paper focuses on the evaluation of the impact associated with various geometrical and material properties on the overall acoustic performance of generic multi-layer thermo-acoustic sources. First, a generalized numerical framework is developed using a state-of-the-art thermo-acoustic emission model for multi-layered devices and is used to forecast the effects associated with different parameters (thickness, density, thermal conductivity, and specific heat capacity), based on a set of 65 536 simulated architectures. Then, the acoustic facility is designed, assembled, and instrumented, and the findings of the simulation campaign are validated against experimental measurements for 32 different samples, manufactured via various vacuum deposition techniques. The results of the experimental campaign corroborate the simulation's prediction and indicate that the variables that have the strongest impact on the thermo-acoustic performance are the thicknesses of the substrate and thermophone layers, as well as the backing's thermal conductivity. Finally, the experimental results are directly comparable with the simulation predictions and the deviation between the two values is within the limits of the experimental accuracy, with an average deviation of 12% (maximal divergence of 28%) and best absolute performance of 0.018 Pa / W when measured from a distance of 75 mm . Overall, the findings provide an insight into the effect of analyzed properties and offer a set of tangible guidelines that can be applied in the future toward the design optimization process that can potentially result in higher-efficiency thermophone-on-substrate thermo-acoustic emitters. [ABSTRACT FROM AUTHOR]

Although matched-field geoacoustic inversion (MFI) has been successfully applied for seabed sediment parameters, its estimation capabilities are limited due to the range dependence of ocean sound-speed profiles, bathymetry, and bottom sediment. This paper proposes a method called propagation invariant (PI). As a purely data-driven method, PI eliminates the effect of intervening range-dependent variations on acoustic propagation and depends only on the "local" environmental characteristics, thus providing ideal input data for MFI. In this paper, the PI concept is applied for MFI in a range-dependent environment by simulations. Results show good MFI performance even for a highly range-dependent waveguide. [ABSTRACT FROM AUTHOR]

An indoor acoustic scene monitoring system using a periodic impulse signal was previously developed. Compared with the impulse signal, the chirp signal is more robust against environmental noise due to its specific spectro-temporal structure. Such specific structure makes the chirp sound easily detected using a spectro-temporal modulation filtering mechanism. In this paper, we demonstrated a system that employs a two-dimensional spectro-temporal filtering mechanism on a Fourier spectrogram to measure the total energy of the reverberations of the chirp signal as the representation of the acoustic scene. The system compares the reverberation energy difference between consecutive chirps with a predefined threshold to automatically detect the change in the acoustic scene. Simulations were conducted in real living rooms with various types of background noise. Test results demonstrated that the proposed system is much more effective than previously developed systems for detecting the acoustic scene changes due to the intruder silently walking in the rooms. In the biggest test room (18 × 9.8 × 2.5 m3) with heavy background noise, the proposed system can still yield a correct identification rate higher than 80% to the intruder walking at 7 m from the microphone without producing any false alarms. [ABSTRACT FROM AUTHOR]

The acoustic responses of a dynamical system reflect the characteristics of the external excitation and vibrating system. The objective in an identification problem is to characterise the system and excitation from the measured responses. This becomes challenging if the characteristics of the dynamical system are time-varying. The analysis of the acoustic signal generated as a result of the tyre-pavement interaction (TPI) is one such situation, where in order to study the time-varying nature there is a need to obtain the characteristics of the instantaneous response. This study explores the single frequency filtering analysis of signals to examine the time-varying characteristics of tyre-pavement interaction noise (TPIN). Field measurements of TPI acoustic signals were obtained for a two-wheeler driven at different speeds on asphalt and cement concrete pavements. The time-frequency characteristics of the tyre tread impact and of the air cavity resonances are analysed by investigating the effect of vehicular speed and pavement type on TPIN. The specific features of TPIN such as the harmonics due to periodic tread impact and the frequency response due to resonances of the air cavities are examined in detail. The analysis presented in this paper will be useful in developing methods for health monitoring of tyre and pavement systems. [ABSTRACT FROM AUTHOR]

Click-evoked otoacoustic emissions (CEOAEs) are clinically used as an objective way to infer whether cochlear functions are normal. However, because the sound pressure level of CEOAEs is typically much lower than the background noise, it usually takes hundreds, if not thousands, of repetitions to estimate the signal with sufficient accuracy. In this paper, we propose to improve the signal-to-noise ratio (SNR) of CEOAE signals within limited measurement time by optimal shrinkage (OS) in two different settings: covariance-based optimal shrinkage (cOS) and singular value decomposition-based optimal shrinkage (sOS). By simulation, the cOS consistently enhanced the SNR by 1–2 dB from a baseline method that is based on calculating the median. In real data, however, the cOS cannot enhance the SNR over 1 dB. The sOS achieved a SNR enhancement of 2–3 dB in simulation and demonstrated capability to enhance the SNR in real recordings. In addition, the level of enhancement increases as the baseline SNR decreases. An appealing property of OS is that it produces an estimate of all single trials. This property makes it possible to investigate CEOAE dynamics across a longer period of time when the cochlear conditions are not strictly stationary. [ABSTRACT FROM AUTHOR]

This paper deals with the experimental study of an acoustic Parity-Time (PT) symmetric system based on the thermoacoustic amplification process. Such a system is presented and consists of two acoustic units connected through side branches to a waveguide. One unit contains a thermoacoustic core that provides an acoustic gain which balances the thermal and viscous losses taking place in the second unit. Two control parameters are set to adjust the impedance of the two units and thereby achieve the PT-symmetry condition. The results show that a good balance between gain and loss is achieved within a frequency range from 45 Hz to 60 Hz. The spontaneous PT-symmetry breaking and the existence of exceptional points, which are characteristic of the behavior of PT-symmetric systems, are explored in this frequency range. Moreover, the distance between the two units is shown to be a control parameter to slightly shift the frequency at which the exceptional points occur. [ABSTRACT FROM AUTHOR]

Target localization can be accomplished through an ultrasonic sonar system equipped with an emitter and two receivers. Time of flight of the sonar echoes allows the calculation of the distance of the target. The orientation can be estimated from knowledge of the beam pattern of the receivers and the ratio, in the frequency domain, between the emitted and the received signals after compensation for distance effects and air absorption. The localization method is described and, as its performance strongly depends on the beam pattern, the search of the most appropriate sonar receiver in order to ensure the highest accuracy of target orientation estimations is developed in this paper. The structure designs considered are inspired by the ear shapes of some bat species. Parameters like flare rate, truncation angle, and tragus are considered in the design of the receiver structures. Simulations of the localization method allow us to state which combination of those parameters could provide the best real world implementation. Simulation results show the estimates of target orientations are, in the worst case, 2° with SNR = 50 dB using the receiver structure chosen for a potential practical implementation of a sonar system. [ABSTRACT FROM AUTHOR]

A protruding noseleaf and concave pinna structures suggest that some bats may use these to enhance their echolocation capabilities. This paper considers two possible mechanisms that each exploit the combination of direct and delayed acoustic paths to achieve more complex emission or sensitivity echolocation patterns. The first is an emission mechanism, in which the protruding noseleaf vibrates to emit sound in both the forward and backward directions, and pinna structures reflect the backward emission to enhance the forward beam. The second is a reception mechanism, which has a direct echo path to the ear canal and a delayed path involving pinna structures reflecting onto the noseleaf and then into the ear canal. A model using Davis' Round-eared Bat illustrates that such direct and delayed acoustic paths provide target elevation cues. The model demonstrates the delayed pinna component can increase the on-axis emission strength, narrow the beam width, and sculpt frequency-dependent beam patterns useful for echolocation. [ABSTRACT FROM AUTHOR]

Over the past decade there has been a great amount of research effort devoted to the topic of acoustic metamaterials (AMMs). The recent development of AMMs has enlightened the way of manipulating sound waves. Several potential applications such as low-frequency noise reduction, cloaking, angular filtering, subwavelength imaging, and energy tunneling have been proposed and implemented by the so-called membrane- or plate-type AMMs. This paper aims to offer a thorough overview on the recent development of membrane- or plate-type AMMs. The underlying mechanism of these types of AMMs for tuning the effective density will be examined first. Four different groups of membrane- or plate-type AMMs (membranes with masses attached, plates with masses attached, membranes or plates without masses attached, and active AMMs) will be reviewed. The opportunities, limitations, and challenges of membrane- or plate-type AMMs will be also discussed. [ABSTRACT FROM AUTHOR]

A data-based matched-mode source localization method is proposed in this paper for a moving source, using mode wavenumbers and depth functions estimated directly from the data, without requiring any environmental acoustic information and assuming any propagation model. The method is in theory free of the environmental mismatch problem because the mode replicas are estimated from the same data used to localize the source. Besides the estimation error due to the approximations made in deriving the data-based algorithms, the method has some inherent drawbacks: (1) It uses a smaller number of modes than theoretically possible because some modes are not resolved in the measurements, and (2) the depth search is limited to the depth covered by the receivers. Using simulated data, it is found that the performance degradation due to the afore-mentioned approximation/limitation is marginal compared with the original matched-mode source localization method. The proposed method has a potential to estimate the source range and depth for real data and be free of the environmental mismatch problem, noting that certain aspects of the (estimation) algorithms have previously been tested against data. The key issues are discussed in this paper. [ABSTRACT FROM AUTHOR]

Matched field processing is a model-based framework for localizing targets in complex propagation environments. In underwater acoustics, it has been extensively studied for improving localization performance in multimodal and multipath media. For guided wave structural health monitoring problems, matched field processing has not been widely applied but is an attractive option for damage localization due to equally complex propagation environments. Although effective, matched field processing is often challenging to implement because it requires accurate models of the propagation environment, and the optimization methods used to generate these models are often unreliable and computationally expensive. To address these obstacles, this paper introduces data-driven matched field processing, a framework to build models of multimodal propagation environments directly from measured data, and then use these models for localization. This paper presents the data-driven framework, analyzes its behavior under unmodeled multipath interference, and demonstrates its localization performance by distinguishing two nearby scatterers from experimental measurements of an aluminum plate. Compared with delay-based models that are commonly used in structural health monitoring, the data-driven matched field processing framework is shown to successfully localize two nearby scatterers with significantly smaller localization errors and finer resolutions. [ABSTRACT FROM AUTHOR]

This article focuses on computations of low frequency sound propagation from an off-shore wind farm. Two different methods for sound propagation calculations are combined with meteorological data for every 3 hours in the year 2010 to examine the varying noise levels at a reception point at 13 km distance. It is shown that sound propagation conditions play a vital role in the noise impact from the off-shore wind farm and ordinary assessment methods can become inaccurate at longer propagation distances over water. Therefore, this paper suggests that methodologies to calculate noise emission with realistic sound speed profiles need to be combined with meteorological data over extended time periods to evaluate the impact of low frequency noise from modern off-shore wind farms. [ABSTRACT FROM AUTHOR]

Plates reinforced by ribs or joists are common elements in lightweight building structures, as well as in other engineering structures such as vehicles, ships, and aircraft. These structures, however, are often not well suited for simple structural acoustic prediction models such as statistical energy analysis. One reason is that the modal density is not uniformly distributed due to the spatial periodicity introduced by the ribs. This phenomenon is investigated in the present paper, using a modal model of a ribbed plate. The modal model uses the Fourier sine modes, and the coupling between the plate and ribs is incorporated using Hamilton's principle. This model is then used to investigate the modal density of the considered spatially periodic structure, and a grouping of the modes in different dominating directions is proposed. Suggestions are also given regarding how to proceed towards a simplified prediction model for ribbed plates. [ABSTRACT FROM AUTHOR]

The paper presents an application of a method for the characterization of underwater acoustic signals based on the statistics of the wavelet transform sub-band coefficients in range-dependent environments. As it was illustrated in previous work, this statistical characterization scheme is a very efficient tool for obtaining observables to be exploited in problems of ocean acoustic tomography and geoacoustic inversion, when range-independent environments are considered. Now the scheme is applied in range-dependent environments for the estimation of range-dependent features in shallow water. A simple denoising strategy, also presented in the paper, is shown to enhance the quality of the inversion results as it helps to keep the signal characterization to the energy a significant part of it. The results presented for typical test cases are encouraging and indicative of the potential of the method for the treatment of inverse problems in acoustical oceanography. [ABSTRACT FROM AUTHOR]

An imaging method of acoustic spinning modes propagating within a circular duct simply with surface pressure information is introduced in this paper. The proposed method is developed in a theoretical way and is demonstrated by a numerical simulation case. Nowadays, the measurements within a duct have to be conducted using in-duct microphone array, which is unable to provide information of complete acoustic solutions across the test section. The proposed method can estimate immeasurable information by forming a so-called observer. The fundamental idea behind the testing method was originally developed in control theory for ordinary differential equations. Spinning mode propagation, however, is formulated in partial differential equations. A finite difference technique is used to reduce the associated partial differential equations to a classical form in control. The observer method can thereafter be applied straightforwardly. The algorithm is recursive and, thus, could be operated in real-time. A numerical simulation for a straight circular duct is conducted. The acoustic solutions on the test section can be reconstructed with good agreement to analytical solutions. The results suggest the potential and applications of the proposed method. [ABSTRACT FROM AUTHOR]

Among noise control techniques, enclosures are widely used. It is known that enclosure acoustic efficiency is strongly influenced by the presence of openings or leaks. Modeling of diffuse field sound transmission loss (TL) of apertures and slits is therefore critical when the enclosure acoustic performance characteristics need to be predicted with confidence either for design or for modifying existing enclosures. Recently, a general model for diffuse field sound TL of rectangular and circular apertures has been developed and validated with respect to existing analytical or numerical models. This paper presents an experimental validation of this new model. The aim was to develop a simple, reliable tool for predicting enclosure insertion loss using statistical energy analysis. Twelve out of the 15 test configurations were found to be reliable and were compared with theoretical models, which in fact correlate closely (without adjustment) with the experimental work. [ABSTRACT FROM AUTHOR]

Since conventional silencers in acoustic ducts have problems of size limitations at low frequencies and being prone to high backpressure, locally resonant aluminum patches are introduced in acoustic duct walls aiming at creating frequency stop bands in the low frequency region (below 1 KHz). With these flush mounted patches, promising noise reductions, with no such drawbacks, can be obtained, building on local resonance phenomenon implemented in acoustic metamaterials techniques. The objective of the current paper is to experimentally validate the performance of an array of flexible side-wall-mounted patches inside ducts. The experimental results are compared with Analytical Green's function method as well as Numerical Finite Element Method and a close agreement was found. The results show that the presence of the patches singly or periodically can play a prominent role in designing any acoustic bandgap materials. The effect of the arrays of patches on the effective dynamic density and bulk modulus has also been investigated. [ABSTRACT FROM AUTHOR]

Acoustic antennas have long been utilized to directionally steer acoustic waves in both air and water. Typically, these antennas are comprised of arrays of active acoustic elements, which are electronically phased to steer the acoustic profile in the desired direction. A new technology, known as an acoustic leaky wave antenna (LWA), has recently been shown to achieve directional steering of acoustic waves using a single active transducer coupled to a transmission line passive aperture. The LWA steers acoustic energy by preferential coupling to an input frequency and can be designed to steer from backfire to endfire, including broadside. This paper provides an analysis of resolution as a function of both input frequency and antenna length. Additionally, the resolution is compared to that achieved using an array of active acoustic elements. [ABSTRACT FROM AUTHOR]

A transmission-line acoustic metamaterial is an engineered, periodic arrangement of relatively small unit-cells, the acoustic properties of which can be manipulated to achieve anomalous physical behaviours. These exotic properties open the door to practical applications, such as an acoustic leaky-wave antenna, through the implementation of radiating channels along the metamaterial. In the transmitting mode, such a leaky-wave antenna is capable of steering sound waves in frequency-dependent directions. Used in reverse, the antenna presents a well defined direction-frequency behaviour. In this paper, an acoustic leaky-wave structure is presented in the receiving mode. It is shown that it behaves as a sound source direction-finding device using only one sensor. After a general introduction of the acoustic leaky-wave antenna concept, its radiation pattern and radiation efficiency are expressed in closed form. Then, numerical simulations and experimental assessments of the proposed transmission-line based structure, implementing only one sensor at one termination, are presented. It is shown that such a structure is capable of finding the direction of an incoming sound wave, from backward to forward, based on received sound power spectra. This introduces the concept of sound source localization without resorting to beam-steering techniques based on multiple sensors. [ABSTRACT FROM AUTHOR]

This paper deals with nonporous windscreens used for reducing noise in infrasonic measurements. A model of sound transmission using a modal approach is derived. The system is a square plate coupled with a cavity. The model agrees with finite element simulations and measurements performed on two windscreens: a cubic windscreen using a material recommended by Shams, Zuckerwar, and Sealey [J. Acoust. Soc. Am. 118, 1335-1340 (2005)] and an optimized flat windscreen made out of aluminum. Only the latter was found to couple acoustical waves below 10 Hz without any attenuation. Moreover, wind noise reduction measurements show that nonporous windscreens perform similarly as a pipe array by averaging the pressure fluctuations. These results question the assumptions of Shams et al. and Zuckerwar [J. Acoust. Soc. Am. 127, 3327-3334 (2010)] about compact nonporous windscreens design and effectiveness. [ABSTRACT FROM AUTHOR]

The acoustic vector-sensor (a.k.a. the vector hydrophone) is a practical and versatile soundmeasurement device, with applications in-room, open-air, or underwater. It consists of three identical uni-axial velocity-sensors in orthogonal orientations, plus a pressure-sensor--all in spatial collocation. Its far-field array manifold [Nehorai and Paldi (1994). IEEE Trans. Signal Process. 42, 2481-2491; Hawkes and Nehorai (2000). IEEE Trans. Signal Process. 48, 2981-2993] has been introduced into the technical field of signal processing about 2 decades ago, and many direction-finding algorithms have since been developed for this acoustic vector-sensor. The above array manifold is subsequently generalized for outside the far field in Wu, Wong, and Lau [(2010). IEEE Trans. Signal Process. 58, 3946-3951], but only if no reflection-boundary is to lie near the acoustic vector-sensor. As for the near-boundary array manifold for the general case of an emitter in the geometric near field, the far field, or anywhere in between--this paper derives and presents that array manifold in terms of signal-processing mathematics. Also derived here is the corresponding Cramér-Rao bound for azimuth-elevation-distance localization of an incident emitter, with the reflected wave shown to play a critical role on account of its constructive or destructive summation with the line-of-sight wave. The implications on source localization are explored, especially with respect to measurement model mismatch in maximum-likelihood direction finding and with regard to the spatial resolution between coexisting emitters. [ABSTRACT FROM AUTHOR]

Most of acoustical experiments face multipath propagation issues. The times of arrival of different ray paths on a sensor can be very close. To estimate them, high resolution algorithms have been developed. The main drawback of these methods is their need of a full rank spectral matrix of the signals. The frequential smoothing technique overcomes this issue by dividing the received signal spectrum into several overlapping sub-bands. This division yields a transfer matrix that may suffer rank deficiency. In this paper, a new criterion to optimally choose the sub-band frequencies is proposed. Encouraging results were obtained on real-world data. [ABSTRACT FROM AUTHOR]

In a previous paper [Ménard et al., J. Acoust. Soc. Am. 126, 1406-1414 (2009)], it was demonstrated that, despite enhanced auditory discrimination abilities for synthesized vowels, blind adult French speakers produced vowels that were closer together in the acoustic space than those produced by sighted adult French speakers, suggesting finer control of speech production in the sighted speakers. The goal of the present study is to further investigate the articulatory effects of visual deprivation on vowels produced by 11 blind and 11 sighted adult French speakers. Synchronous ultrasound, acoustic, and video recordings of the participants articulating the ten French oral vowels were made. Results show that sighted speakers produce vowels that are spaced significantly farther apart in the acoustic vowel space than blind speakers. Furthermore, blind speakers use smaller differences in lip protrusion but larger differences in tongue position and shape than their sighted peers to produce rounding and place of articulation contrasts. Trade-offs between lip and tongue positions were examined. Results are discussed in the light of the perception-for-action control theory. [ABSTRACT FROM AUTHOR]

A pressure compensated metal diaphragm based fiber laser hydrophone configuration that can provide good sensitivity, large bandwidth, and sea state zero noise floor is proposed in this paper. A simplified theoretical model of the proposed sensor configuration is developed in which the acoustic elements of the sensor configuration are modeled using a four-pole acoustic transfer matrix and the structural elements are modeled as second order single degree of freedom elements. This model is then used to optimize the design parameters of the sensor system to achieve the performance objectives. An axisymmetric finite element analysis of the sensor configuration is also carried out to validate the results from the simplified theoretical model. Prototype sensors were fabricated and hydrostatic testing in a pressure vessel validated the static pressure compensation performance of the sensor. Frequency dependent sensitivity of the sensor system was measured through acoustic testing in a water tank. The prototype sensor gave a flat frequency response up to 5 kHz and experimental results compared well with theoretical predictions. The sensor has an acceleration rejection figure on the order of 0 dB ref 1 m/s2 Pa and the pressure compensation approach worked reasonably well up to a hydrostatic pressures equivalent to a depth of 50 m. [ABSTRACT FROM AUTHOR]

The knowledge of parameters characterizing an acoustic environment, such as the geometric information about a room, can be used to enhance the performance of several audio applications. In this paper, a novel method for three-dimensional room geometry inference based on robust and high-resolution beamforming techniques for spherical microphone arrays is presented. Unlike other approaches that are based on the measurement and processing of multiple room impulse responses, here, microphone array signal processing techniques for uncontrolled broadband acoustic signals are applied. First, the directions of arrival (DOAs) and time differences of arrival (TDOAs) of the direct signal and room reflections are estimated using high-resolution robust broadband beamforming techniques and cross-correlation analysis. In this context, the main challenges include the low reflected-signal to background-noise power ratio, the low energy of reflected signals relative to the direct signal, and their strong correlation with the direct signal and among each other. Second, the DOA and TDOA information is combined to infer the room geometry using geometric relations. The high accuracy of the proposed room geometry inference technique is confirmed by experimental evaluations based on both simulated and measured data for moderately reverberant rooms. [ABSTRACT FROM AUTHOR]

In a noisy environment, the sound field of a source is composed of three parts, which are: The field that would be radiated by the target source into free space, the incoming field from disturbing sources or reflections, and the scattered field that is created by the incoming wave falling on the target source. To accurately identify the sound source with nearfield acoustic holography in that situation, the last two parts must be removed from the mixed field. In a previous study, a method for recovering the free sound field in a noisy environment was proposed based on the equivalent source method and measurements of pressure [J. Acoust. Soc. Am. 131(2), 1260-1270 (2012)]. In the present paper, that method was modified by allowing the input data to be measurements of particle velocity instead of pressure. An experiment was carried out to examine both the pressure- and velocity-based methods, and the performance of the two methods was compared. It was found that both methods are capable of reconstructing the free-field pressure radiated by the target source based on measurements made in a noisy environment, but the velocity-based method shows a large benefit in the reconstruction of the free-field particle velocity. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging offers significant potential as a biomedical imaging modality. For some applications, however, there is a need for contrast enhancement. In this paper, a theoretical comparison is presented of the efficacy of three different designs for photoacoustic contrast agents (PACAs), specifically, a droplet of dye, a bubble filled with gas coated by a dye loaded shell, and a droplet of volatile dye. For each case, the governing equations describing the dynamics of a single PACA in a homogenous incompressible fluid are derived. The coupled sets of equations describing the bubble oscillation and resulting radiated pressure, the photo-acoustic energy equation, and the equation of state are then solved numerically. The numerical results predict a stronger radiated acoustic signal for the same optical source energy density in the case of the volatile dye droplet by a factor of up to two orders of magnitude compared with the other two types of agent. [ABSTRACT FROM AUTHOR]

The process of echolocation is accomplished by bats partly using the beam profiles associated with their ear shapes that allow for discrimination between different echo directions. Indeed, knowledge of the emitted signal characteristic and measurement of the echo travel time from a target make it possible to compensate for attenuation due to distance, and to focus on filtering through the receivers' beam profiles by comparing received echoes to the original signal at all frequencies in the spectrum of interest. From this basis, a beam profile method to localize a target in three-dimensional space for an ultrasonic sensor system equipped with an emitter and two receivers is presented. Simulations were conducted with different noise levels, and only the contribution of the receivers' beam profiles was considered to estimate the orientation of the target with respect to the receivers. The beam pattern of the Phyllostomus discolor's ear was adopted as that of a receiver. Analyses of beam resolution and frequency ranges were conducted to enhance the accuracy of orientation estimates. The choice of appropriate resolution and frequency ranges guarantee that error mean values for most of the orientations are within [0.5°, 1.5°], even in noisy situations: Signal-to-noise ratio values considered in this work are 35 and 50 dB. [ABSTRACT FROM AUTHOR]

This paper presents a method to identify the surface areas of a vibrating structure that contribute to the radiated sound power. The surface contributions of the structure are based on the acoustic radiation modes and are computed for all boundaries of the acoustic domain. The surface contributions are compared to the acoustic intensity, which is a common measure for near-field acoustic energy. Sound intensity usually has positive and negative values that correspond to energy sources and sinks on the surface of the radiating structure. Sound from source and sink areas partially cancel each other and only a fraction of the near-field acoustic energy reaches the far-field. In contrast to the sound intensity, the surface contributions are always positive and no cancelation effects exist. The technique presented here provides a method to localize the relevant radiating surface areas on a vibrating structure. To illustrate the method, the radiated sound power from a baffled square plate is presented. [ABSTRACT FROM AUTHOR]

This paper investigates the sound transmission loss (STL) of aeroelastic plates reinforced by two sets of orthogonal rib-stiffeners in the presence of external mean flow. Built upon the periodicity of the structure, a comprehensive theoretical model is developed by considering the convection effect of mean flow. The rib-stiffeners are modeled by employing the Bernoulli-Euler beam theory and the torsional wave equation. While the solution for the transmission loss of the structure based on plate displacement and acoustic pressures is given in the form of space-harmonic series, the corresponding coefficients are obtained from the solution of a system of linear equations derived from the plate-beam coupling vibration governing equation and Helmholtz equation. The model predictions are validated by comparing with existing theoretical and experimental results in the absence of mean flow. A parametric study is subsequently performed to quantify the effects of mean flow as well as structure geometrical parameters upon the transmission loss. It is demonstrated that the transmission loss of periodically rib-stiffened structure is increased significantly with increasing Mach number of mean flow over a wide frequency range. The STL value for the case of sound wave incident downstream is pronouncedly larger than that associated with sound wave incident upstream. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the acoustic emissions emitted by an underway REMUS-100 autonomous underwater vehicle (AUV) that were obtained near Honolulu Harbor, HI using a fixed, bottom-mounted horizontal line array (HLA). Spectral analysis, beamforming, and cross-correlation facilitate identification of independent sources of noise originating from the AUV. Fusion of navigational records from the AUV with acoustic data from the HLA allows for an aspect-dependent presentation of calculated source levels of the strongest propulsion tone. [ABSTRACT FROM AUTHOR]

This paper presents an imaging method for the localization of the impact point in complex anisotropic structures with diffuse field conditions, using only one passive transducer. The proposed technique is based on the reciprocal time reversal approach (inverse filtering) applied to a number of waveforms stored into a database containing the experimental Green's function of the structure. Unlike most acoustic emission monitoring systems, the present method exploits the benefits of multiple scattering, mode conversion, and boundaries reflections to achieve the focusing of the source with high resolution. Compared to a standard time reversal approach, the optimal refocusing of the back propagated wave field at the impact point is accomplished through a 'virtual' imaging process. The robustness of the inverse filtering technique is experimentally demonstrated on a dissipative stiffened composite panel and the source position can be retrieved with a high level of accuracy in any position of the structure. Its very simple configuration and minimal processing requirements make this method a valid alternative to the conventional imaging Structural Health Monitoring systems for the acoustic emission source localization. [ABSTRACT FROM AUTHOR]

Industrial noise can be successfully mitigated with the combined use of passive and active noise control (ANC) strategies. In a noisy area, a practical solution for noise attenuation may include both the use of baffles and ANC. When the operator is required to stay in movement in a delimited spatial area, conventional ANC is usually not able to adequately cancel the noise over the whole area. Control strategies need to be devised to achieve acceptable spatial coverage. A three-dimensional vibration-acoustic model is proposed in this paper. The signal of an accelerometer attached to the bulk of a centrifugal pump installed in an empty room was used as the input of this model. The signal of a microphone that changes its position in a spatial grid inside this room is the output. In the first stage, the ARX models are used to describe a SISO system where the input is the machine vibration and the output is the noise level measured at a certain point. In the second stage, spatial interpolation is used to estimate the model parameters. Results show good agreement between experimental data and model predictions, indicating the potential of using the model for the design of ANC. [ABSTRACT FROM AUTHOR]

A method is presented for passive imaging of cavitational acoustic emissions using an ultrasound array, with potential application in real-time monitoring of ultrasound ablation. To create such images, microbubble emissions were passively sensed by an imaging array and dynamically focused at multiple depths. In this paper, an analytic expression for a passive image is obtained by solving the Rayleigh–Sommerfield integral, under the Fresnel approximation, and passive images were simulated. A 192-element array was used to create passive images, in real time, from 520-kHz ultrasound scattered by a 1-mm steel wire. Azimuthal positions of this target were accurately estimated from the passive images. Next, stable and inertial cavitation was passively imaged in saline solution sonicated at 520 kHz. Bubble clusters formed in the saline samples were consistently located on both passive images and B-scans. Passive images were also created using broadband emissions from bovine liver sonicated at 2.2 MHz. Agreement was found between the images and source beam shape, indicating an ability to map therapeutic ultrasound beams in situ. The relation between these broadband emissions, sonication amplitude, and exposure conditions are discussed. [ABSTRACT FROM AUTHOR]

This paper analyzes the chaotic ray dynamics at multimegater ranges in a deep water environment with internal-wave-induced fluctuations of the sound speed. The behavior of acoustic ray paths is investigated using the Hamiltonian formalism expressed in terms of action-angle variables. It is shown that the range dependence of the action variable of chaotic ray can be approximated by a random Wiener process. On the basis of this result an approximate statistical description of the chaotic ray structure is derived. Distributions of coordinates, momenta (grazing angles), and actions of sound rays are evaluated. This statistical approach is used for studying ray travel times, that is, arrival times of sound pulses coming to the receiver through different ray paths. The spread of travel times for a bundle of rays with close starting parameters and the influence of sound speed fluctuations on the timefront representing ray arrivals in the time-depth plane are examined. Estimates for the widening and bias of the timefront segment caused by the fluctuations are obtained. [ABSTRACT FROM AUTHOR]

This paper presents a high-order accelerated algorithm for the solution of the integral-equation formulation of volumetric scattering problems. The scheme is particularly well suited to the analysis of “thin” structures as they arise in certain applications (e.g., material coatings); in addition, it is also designed to be used in conjunction with existing low-order FFT-based codes to upgrade their order of accuracy through a suitable treatment of material interfaces. The high-order convergence of the new procedure is attained through a combination of changes of parametric variables (to resolve the singularities of the Green function) and “partitions of unity” (to allow for a simple implementation of spectrally accurate quadratures away from singular points). Accelerated evaluations of the interaction between degrees of freedom, on the other hand, are accomplished by incorporating (two-face) equivalent source approximations on Cartesian grids. A detailed account of the main algorithmic components of the scheme are presented, together with a brief review of the corresponding error and performance analyses which are exemplified with a variety of numerical results. [ABSTRACT FROM AUTHOR]

Acoustic communications and positioning are vital aspects of unmanned underwater vehicle operations. The usage of separate units on each vehicle has become an issue in terms of frequency bandwidth, space, power, and cost. Most vehicles rely on acoustic modems transmitting frequency-hopped multiple frequency-shift keyed sequences for command-and-control operations, which can be used to locate the vehicle with a good level of accuracy without requiring extra signal transmission. In this paper, an ultrashort baseline acoustic positioning technique has been designed, simulated, and tested to locate an acoustic modem source in three dimensions using a tetrahedral, half-wavelength acoustic antenna. The position estimation is performed using the detection sequence contained in each message, which is a series of frequency-hopped pulses. Maximum likelihood estimation of azimuth and elevation estimation is performed using a varying number of pulse and various signal-to-noise ratios. Simulated and measured position estimation error match closely, and indicate that the accuracy of this system improves dramatically as the number of pulses processed increases, given a fixed signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The design of transducers to excite and detect guided waves is a fundamental part of a nondestructive evaluation or structural health monitoring system and requires the ability to predict the radiated guided wave field of a transmitting transducer. For most transducers, this can be performed by making the assumption that the transducer is weakly coupled and then integrating the Green’s function of the structure over the area of the transducer. The majority of guided wave modeling is based on two-dimensional (2D) formulations where plane, straight-crested waves are modeled. Several techniques can be readily applied to obtain the solution to the forced 2D problem in terms of modal amplitudes. However, for transducer modeling it is desirable to obtain the complete three-dimensional (3D) field, which is particularly challenging in anisotropic materials. In this paper, a technique for obtaining a far-field asymptotic solution to the 3D Green’s function in terms of the modal solutions to the forced 2D problem is presented. Results are shown that illustrate the application of the technique to isotropic (aluminium) and anisotropic (cross-ply and unidirectional composite) plates. Where possible, results from the asymptotic model are compared to those from 3D time-marching finite element simulations and good agreement is demonstrated. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for the off-line reproduction of random pressure fields with given spatial correlation characteristics. The simulation method is first presented, together with an outline of the signal processing techniques required. The design of an experimental setup is then detailed in relation with the nature of the simulated pressure fields. Of particular interest is the laboratory synthesis of three different types of partially correlated random excitations: an acoustic diffuse field, a grazing incident plane wave, and a turbulent boundary layer. The corresponding excitations are generated in a semianechoic chamber over a test panel using a near-field array of 16 loudspeakers driven by a set of optimal signals. The loudspeakers are positioned at a suitable distance above a sufficiently dense grid of microphones close to the simulation surface. The mutually correlated drive signals are determined from both the target correlation properties and the acoustic transfer functions measured between the loudspeakers and the microphones. This approach could provide a cost-effective method of reducing the variation of low frequency sound transmission measurements as well as simulating the propeller-induced noise and the boundary layer noise transmitted through aircraft fuselage structures. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) data from single point turning machining are analyzed in this paper in order to gain a greater insight of the signal statistical properties for tool condition monitoring applications. A statistical analysis of the time series data amplitude and root mean square (rms) value at various tool wear levels are performed, finding that aging features can be revealed in all cases from the observed experimental histograms. In particular, AE data amplitudes are shown to be distributed with a power-law behavior above a crossover value. An analytic model for the rms values probability density function is obtained resorting to the Jaynes' maximum entropy principle; novel technique of constraining the modeling function under few fractional moments, instead of a greater amount of ordinary moments, leads to well-tailored functions for experimental histograms. [ABSTRACT FROM AUTHOR]

The rapidly warming Arctic ocean demands new ways to monitor and characterize changes in sea-ice distribution, thickness, and mechanical properties. Upward-looking sonars mounted on autonomous underwater vehicles offer possibilities for doing so. Numerical simulations were made of the signal received by an upward-looking sonar under a smooth ice sheet using a wavenumber integration code. Demands on sonar frequency and bandwidth for pulse-echo measurements were analyzed. For typical sea-ice physical properties found in the Arctic ocean, even in highly attenuating sea-ice, there is significant information to be extracted from the received acoustic signal. Discrete resonance frequencies in the signal may be related to leaky Lamb waves, and the frequencies are connected to the ratio of the shear wave speed-to-thickness of the ice sheet. The periodicity of the multiple reflections of a pulse-compressed signal may be related to the ratio of compressional wave speed-to- thickness. Decay rates of both types of signals are indicative of the wave attenuation coefficients. Simulations of the acoustic reflection by rough water–ice interfaces were made. Smaller levels of roughness were found to enhance the acoustic signal, while greater levels of roughness are detrimental to the sea-ice characterization process. [ABSTRACT FROM AUTHOR]

Various physical characteristics, including ultrasonic waves, active acoustic emissions, vibrations, and thermal imaging, have been used for blade fault detection. In this work, we propose using the sound produced by spinning wind blades to identify faults. To the best of our knowledge, passive acoustic information has not yet been explored for this task. In particular, we develop three networks targeting different scenarios. The main contributions of this work are threefold. First, when normal and aberrant data are available for supervised learning, an attention-convolutional recurrent neural network is designed to show the feasibility of using passive sound information to conduct fault detection. Second, in the absence of abnormal training data, we build a normal-encoder network to learn the distributions of normal data through semisupervised learning, which avoids the requirement of abnormal training data. Third, when multiple devices are used to collect the data, due to different properties of devices, there is a domain mismatch issue. To overcome this, we create an adversarial domain adaptive network to close the gap between the source and target domains. Acoustic signal datasets of actual wind turbine operations are collected to evaluate our fault detection systems. The findings demonstrate that the proposed systems offer high classification accuracy and indicate the feasibility of passive acoustic signal-based wind turbine blade fault detection with one step close to automatic detection. [ABSTRACT FROM AUTHOR]

Noise generation by low Mach number air flows through circular orifices in rectangular ducts is investigated. In particular, the influence of the number and position of the orifices maintaining a constant flow area is addressed. A review of the available theories suggests a certain importance of such parameters in the excitation of higher-order acoustic duct modes. A qualitative coefficient is proposed for a first characterization of the ability to enhance or lessen a given higher-order acoustic mode by the plate geometry. An experimental campaign is performed to measure the total emitted acoustic power by different plate geometries as well as its modal composition. It is found that the orifices' numbers and positions greatly influence the acoustic emissions while the flow pressure drop caused by the obstacles is similar. The proposed qualitative coefficient shows good agreement with the experimental results. A particle image velocimetry measurement campaign is performed to visualize the near-field average flow behavior upstream and downstream of the orifice plates. An increase in the turbulent velocity fluctuations in the vicinity of the orifices is observed on both sides, further validating previous studies on the subject. [ABSTRACT FROM AUTHOR]

An autonomous surface vehicle known as a wave glider, instrumented with a low-power towed hydrophone array and embedded digital signal processor, is demonstrated as a viable low-noise system for the passive acoustic monitoring of marine mammals. Other key design elements include high spatial resolution beamforming on a 32-channel towed hydrophone array, deep array deployment depth, vertical motion isolation, and bandwidth-efficient real-time acoustic data transmission. Using at-sea data collected during a simultaneous deployment of three wave glider-based acoustic detection systems near Stellwagen Bank National Marine Sanctuary in September 2019, the capability of a low-frequency towed hydrophone array to spatially reject noise and to resolve baleen whale vocalizations from anthropogenic acoustic clutter is demonstrated. In particular, mean measured array gain of 15.3 dB at the aperture design frequency results in a post-beamformer signal-to-noise ratio that significantly exceeds that of a single hydrophone. Further, it is shown that with overlapping detections on multiple collaborating systems, precise localization of vocalizing individuals is achievable at long ranges. Last, model predictions showing a 4× detection range, or 16× area coverage, advantage of a 32-channel towed array over a single hydrophone against the North Atlantic right whale upcall are presented for the continental shelf environment south of Martha's Vineyard. [ABSTRACT FROM AUTHOR]

Passive acoustic inversion techniques for measuring gas flux into the water column have the potential to be a powerful tool for the long-term monitoring and quantification of natural marine seeps and anthropogenic emissions. Prior inversion techniques have had limited precision due to lack of constraints on the initial amplitude of a bubble's excitation following its release into the water column ( R ε 0 i ). R ε 0 i is determined by observing the acoustic signal of bubbles released from sediment in a controlled experiment and its use is demonstrated by quantifying the flux from a volcanic CO2 seep offshore Panarea (Italy), improving the precision by 78%. [ABSTRACT FROM AUTHOR]

The transfer function HV between stapes velocity VS and sound pressure near the tympanic membrane PTM is a descriptor of sound transmission through the middle ear (ME). The ME power transmission efficiency (MEE), the ratio of sound power entering the cochlea to power entering the middle ear, was computed from HV measured in seven chinchilla ears and previously reported measurements of ME input admittance YTM and ME pressure gain GMEP [Ravicz and Rosowski, J. Acoust. Soc. Am. 132, 2437-2454 (2012); J. Acoust. Soc. Am. 133, 2208-2223 (2013)] in the same ears. The ME was open, and a pressure sensor was inserted into the cochlear vestibule for most measurements. The cochlear input admittance YC computed from HV and GMEP is controlled by a combination of mass and resistance and is consistent with a minimum-phase system up to 27 kHz. The real part Re{YC}, which relates cochlear sound power to inner-ear sound pressure, decreased gradually with frequency up to 25 kHz and more rapidly above that. MEE was about 0.5 between 0.1 and 8 kHz, higher than previous estimates in this species, and decreased sharply at higher frequencies. [ABSTRACT FROM AUTHOR]

The propagation of weakly dispersive modal pulses is investigated using data collected during the 2004 long-range ocean acoustic propagation experiment (LOAPEX). Weakly dispersive modal pulses are characterized by weak dispersion- and scattering-induced pulse broadening; such modal pulses experience minimal propagation-induced distortion and are thus well suited to communications applications. In the LOAPEX environment modes 1, 2, and 3 are approximately weakly dispersive. Using LOAPEX observations it is shown that, by extracting the energy carried by a weakly dispersive modal pulse, a transmitted communications signal can be recovered without performing channel equalization at ranges as long as 500 km; at that range a majority of mode 1 receptions have bit error rates (BERs) less than 10%, and 6.5% of mode 1 receptions have no errors. BERs are estimated for low order modes and compared with measurements of signal-to-noise ratio (SNR) and modal pulse spread. Generally, it is observed that larger modal pulse spread and lower SNR result in larger BERs. [ABSTRACT FROM AUTHOR]

Ocean bottom seismometer observations at 5000 m depth during the long-range ocean acoustic propagation experiment in the North Pacific in 2004 show robust, coherent, late arrivals that are not readily explained by ocean acoustic propagation models. These 'deep seafloor' arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km. The travel times for six (of 16 observed) deep seafloor arrivals correspond to the sea surface reflection of an out-of-plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers. This out-of-plane bottom-diffracted surface-reflected energy is observed on the deep vertical line array about 35 dB below the peak amplitude arrivals and was previously misinterpreted as in-plane bottom-reflected surface-reflected energy. The structure of these arrivals from 500 to 3200 km range is remarkably robust. The bottom-diffracted surface-reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor. [ABSTRACT FROM AUTHOR]

Mode travel time estimation in the presence of internal waves (IWs) is a challenging problem. IWs perturb the sound speed, which results in travel time wander and mode scattering. A standard approach to travel time estimation is to pulse compress the broadband signal, pick the peak of the compressed time series, and average the peak time over multiple receptions to reduce variance. The peak-picking approach implicitly assumes there is a single strong arrival and does not perform well when there are multiple arrivals due to scattering. This article presents a statistical model for the scattered mode arrivals and uses the model to design improved travel time estimators. The model is based on an Empirical Orthogonal Function (EOF) analysis of the mode time series. Range-dependent simulations and data from the Long-range Ocean Acoustic Propagation Experiment (LOAPEX) indicate that the modes are represented by a small number of EOFs. The reduced-rank EOF model is used to construct a travel time estimator based on the Matched Subspace Detector (MSD). Analysis of simulation and experimental data show that the MSDs are more robust to IW scattering than peak picking. The simulation analysis also highlights how IWs affect the mode excitation by the source. [ABSTRACT FROM AUTHOR]